How mortgage overpayments work
An overpayment is any amount you pay above your contractual monthly payment. Because interest is charged daily on your outstanding balance, every pound of overpayment immediately stops accruing interest for the rest of the term. The saving compounds: the lower balance means less interest next month, which means more of next month's payment goes on capital, which lowers the balance further.
On a £200,000 mortgage at 4.5% over 25 years, overpaying just £100 a month saves around £19,000 in interest and clears the mortgage about 2 years and 9 months early. Doubling that to £200 a month saves around £32,000 and takes roughly 4 years and 7 months off the term. Early overpayments are the most powerful — a £10,000 lump sum in year 1 saves far more interest than the same £10,000 in year 15.
Monthly vs lump sum overpayments
Regular monthly overpayments are easiest to build into a budget — set up a standing order for £50, £100, or £250 above your direct debit and you'll barely notice. This is usually the lowest-friction path, and because you're paying every month the interest saving starts immediately.
Lump-sum overpayments are better suited to windfalls: an annual bonus, a tax refund, or an inheritance. A single £10,000 lump early in the term can save almost as much interest as five years of modest monthly overpayments — provided it sits inside your lender's annual overpayment allowance.
Many borrowers use both: a small monthly overpayment as the baseline, topped up with a lump sum when a bonus lands. The calculator above lets you model either approach — switch between monthly and lump-sum modes to compare.
Things to check before overpaying
- Early repayment charges (ERCs): most fixed-rate deals allow 10% of the balance per year as overpayment. Paying more usually triggers a 1–5% ERC on the excess.
- Emergency fund first: keep 3–6 months of essential expenses in an easy-access account before diverting cash into the mortgage. Money in a mortgage is not easy to get back in a hurry.
- Compare to savings rates after tax: if a cash ISA pays more than your mortgage rate (tax-free), saving may beat overpaying. A basic-rate taxpayer also gets a £1,000 Personal Savings Allowance on non-ISA interest.
- Pension contributions first: for higher-rate taxpayers, a pension contribution gets 40% tax relief — typically a better return than overpaying a 4–5% mortgage. Consider maxing employer matching before overpaying.
- Check the overpayment method: some lenders only apply overpayments on the monthly anniversary, so a payment made mid-month may sit without saving interest. Others apply overpayments daily. Ask your lender.